Seam welding is widely known as a technique for joining metal plates together (for example, see Japanese Laid-Open Patent Publication No. 2007-167896). Using seam welding, after stacked metal plates (stacked assembly) are sandwiched between a pair of roller electrodes, a current is applied between the roller electrodes. More specifically, within the stacked assembly, a current pathway is formed along the stacking direction. A current that flows out from the plus electrode is conducted successively through the metal plate in contact with the plus electrode, contact sites between the metal plates, and the metal plate in contact with the minus electrode, whereupon the current reaches the minus electrode.
During application of current, resistance heating (Joule heat) is generated at the contact sites between the metal plates. Thus, melting takes place at such sites.
Thereafter, by the stacked assembly being moved relative to the pair of roller electrodes, the current pathway also moves, so that ultimately, the sites at which resistance heating occurs in the stacked assembly move as well. That is, the current moves away from the sites that have been melted prior to such movement, and therefore, resistance heating of the sites is completed. As a result, the temperature at the sites decreases, whereby the sites become solidified and acquire a solid phase. Such solidified sites are referred to primarily as nuggets.
On the other hand, at the sites corresponding to the newly formed current pathway, in the same manner as described above, the contact sites between the metal plates undergo melting. Thereafter, by sequentially repeating the above-described phenomenon, the metal plates are joined continuously to each other.